Harmonise AnimatedSprite3D and its 2D counterpart

Add the following properties to AnimatedSprite3D:
- `backwards` parameter in `play()`;
- `speed_scale`.

Both classes' internals are more similar, down to the line spacings. They've also been updated to be clearer and less inconsistent (e.g. `!frames.is_valid()` -> `frames.is_null()`, use SceneStringNames instead of CoreStringNames, rename the internal _queue_update to _queue_redraw)

doc/classes/AnimatedSprite3D.xml

@@ -13,8 +13,9 @@
 		<method name="play">
 			<return type="void" />
 			<param index="0" name="anim" type="StringName" default="&amp;&quot;&quot;" />
+			<param index="1" name="backwards" type="bool" default="false" />
 			<description>
-				Plays the animation named [param anim]. If no [param anim] is provided, the current animation is played.
+				Plays the animation named [param anim]. If no [param anim] is provided, the current animation is played. If [param backwards] is [code]true[/code], the animation will be played in reverse.
 			</description>
 		</method>
 		<method name="stop">
@@ -37,6 +38,9 @@
 		<member name="playing" type="bool" setter="set_playing" getter="is_playing" default="false">
 			If [code]true[/code], the [member animation] is currently playing.
 		</member>
+		<member name="speed_scale" type="float" setter="set_speed_scale" getter="get_speed_scale" default="1.0">
+			The animation speed is multiplied by this value.
+		</member>
 	</members>
 	<signals>
 		<signal name="animation_finished">

scene/2d/animated_sprite_2d.cpp

@@ -175,33 +175,38 @@ void AnimatedSprite2D::_notification(int p_what) {
 				if (timeout <= 0) {
 					timeout = _get_frame_duration();
 
-					int fc = frames->get_frame_count(animation);
-					if ((!backwards && frame >= fc - 1) || (backwards && frame <= 0)) {
-						if (frames->get_animation_loop(animation)) {
-							if (backwards) {
-								frame = fc - 1;
-							} else {
-								frame = 0;
-							}
-
-							emit_signal(SceneStringNames::get_singleton()->animation_finished);
-						} else {
-							if (backwards) {
+					int last_frame = frames->get_frame_count(animation) - 1;
+					if (!backwards) {
+						// Forward.
+						if (frame >= last_frame) {
+							if (frames->get_animation_loop(animation)) {
 								frame = 0;
-							} else {
-								frame = fc - 1;
-							}
-
-							if (!is_over) {
-								is_over = true;
 								emit_signal(SceneStringNames::get_singleton()->animation_finished);
+							} else {
+								frame = last_frame;
+								if (!is_over) {
+									is_over = true;
+									emit_signal(SceneStringNames::get_singleton()->animation_finished);
+								}
 							}
+						} else {
+							frame++;
 						}
 					} else {
-						if (backwards) {
-							frame--;
+						// Reversed.
+						if (frame <= 0) {
+							if (frames->get_animation_loop(animation)) {
+								frame = last_frame;
+								emit_signal(SceneStringNames::get_singleton()->animation_finished);
+							} else {
+								frame = 0;
+								if (!is_over) {
+									is_over = true;
+									emit_signal(SceneStringNames::get_singleton()->animation_finished);
+								}
+							}
 						} else {
-							frame++;
+							frame--;
 						}
 					}
 
@@ -259,14 +264,15 @@ void AnimatedSprite2D::_notification(int p_what) {
 
 void AnimatedSprite2D::set_sprite_frames(const Ref<SpriteFrames> &p_frames) {
 	if (frames.is_valid()) {
-		frames->disconnect("changed", callable_mp(this, &AnimatedSprite2D::_res_changed));
+		frames->disconnect(SceneStringNames::get_singleton()->changed, callable_mp(this, &AnimatedSprite2D::_res_changed));
 	}
+
 	frames = p_frames;
 	if (frames.is_valid()) {
-		frames->connect("changed", callable_mp(this, &AnimatedSprite2D::_res_changed));
+		frames->connect(SceneStringNames::get_singleton()->changed, callable_mp(this, &AnimatedSprite2D::_res_changed));
 	}
 
-	if (!frames.is_valid()) {
+	if (frames.is_null()) {
 		frame = 0;
 	} else {
 		set_frame(frame);
@@ -283,7 +289,7 @@ Ref<SpriteFrames> AnimatedSprite2D::get_sprite_frames() const {
 }
 
 void AnimatedSprite2D::set_frame(int p_frame) {
-	if (!frames.is_valid()) {
+	if (frames.is_null()) {
 		return;
 	}
 
@@ -318,7 +324,7 @@ void AnimatedSprite2D::set_speed_scale(double p_speed_scale) {
 
 	speed_scale = MAX(p_speed_scale, 0.0f);
 
-	// We adapt the timeout so that the animation speed adapts as soon as the speed scale is changed
+	// We adapt the timeout so that the animation speed adapts as soon as the speed scale is changed.
 	_reset_timeout();
 	timeout -= elapsed;
 }